Hi. You can only expand one subment at a time. Add the active class to the element you want to expand:

<li>
    <a class="collapsible-header waves-effect arrow-r active">
        <i class="fas fa-chevron-right"></i> 
            Submit blog
        <i class="fas fa-angle-down rotate-icon"></i>
    </a>
    <div class="collapsible-body">
        <ul class="list-unstyled">
            <li>
                <a href="#" class="waves-effect">Submit listing</a>
            </li>
            <li>
                <a href="#" class="waves-effect">Registration form</a>
            </li>
        </ul>
    </div>
</li>

Best, Grzegorz